<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        div{
            width: 200px;
            height: 200px;
            background-color: pink;
        }
    </style>
</head>
<body>
    <div ></div>
    <script>
        //1.获取元素
        var div=document.querySelector('div');
        //2.注册事件 处理程序
        div.onclick=function(){
            // div.style里面的属性 采取驼峰命名法
            this.style.backgroundColor='purple';
            this.style.width="400px";
        }
    </script>
</body>
</html>